1. Field of the Invention
The present invention relates generally to optical systems and, more particularly, to systems and methods for wavelength division multiplexing (WDM).
2. Description of Related Art
Wavelength division multiplexing (WDM) is a scheme for increasing the amount of information carried by an optical fiber. Generally, signals are modulated onto light beams, where each beam has a different wavelength. These different-wavelength beams are combined for transmission over a single, typically long-distance, fiber. At the receiving end, the light is split into the different wavelength beams, and each of these is demodulated to obtain the original signal.
FIG. 1 is a block diagram of a conventional communications system 100 employing WDM. The system 100 includes link source device(s) (LSD(s)) 110, LSD outputs 120, a WDM system 130, WDM system outputs 140, and link destination device(s) (LDD(s)) 150. The LSD(s) 110 provide outputs 120 on a pre-selected media (typically an optical fiber) using a pre-selected modulation. The WDM system 130 receives the outputs 120 and ultimately delivers them remotely as outputs 140 to the LDD(s) 150.
The LSD(s) 110 may include one or more switches, routers and/or add-drop multiplexers (ADMs) configured in various combinations to produce the outputs 120. The switch(es) may include networking or transmission devices configured to send data packets directly to ports associated with given network addresses or to cross connect circuits, or some combination thereof. The router(s) may include networking devices configured to find paths for data packets to be sent from one network to another. Such routers may store and forward messages between networks, for example by picking an expedient route based on the traffic load and/or the number or length of hops required. The ADMs may include devices in optical networks used to add and/or drop SONET, SDH, or other TDM channels. The LSD(s) 110 may produce optical signals (e.g., synchronous optical network (SONET) or Ethernet signals) or electrical signals carrying information to the WDM system.
Similarly, the LDD(s) 150 may include one or more switches, routers and/or ADMs configured in various combinations to receive the outputs 140. As with the LSD(s) 110, the LDD(s) 150 may include one or more switches, routers, and ADMs working in combination to receive and process various signals.
The WDM system 130 includes a multiplexer 132 that receives the outputs 120 into an internal digital format, modulates each input onto a different wavelength, combines the wavelengths, and transmits a single optical signal on a (typically wide-area) fiber 134. The WDM system 130 also includes a demultiplexer 136 that receives the signal from the fiber 134, separates the different wavelengths, and converts the information in the wavelengths into digital inputs 140 for the LDD(s) 150.
In FIG. 1, the signal coding of the LSD outputs 120 and the WDM system outputs 140 is typically standards-based. This allows the WDM system 130 to communicate with LSD(s) 110 and LDD(s)150 made by many different vendors. The WDM system 130 may have interchangeable line cards that support particular standard physical layers, such as SONET, Ethernet, etc. The physical layer signal coding of the outputs 120 and the inputs 140 may be the same or may be different.
By contrast, the modulation and line coding used within the WDM system 130 is typically different from that used to communicate with the LSD(s) 110 and LDD(s) 150. The modulation and line coding used within the WDM system 130 is typically proprietary. Because it is a language only spoken by a single vendor's, or a few vendors', WDM systems, a single vendor or a few vendors must supply both the multiplexer 132 and the demultiplexer 136 at both ends of the fiber 134. Different vendors' WDM systems often do not interoperate for this reason. Hence, the media and modulation used within the WDM system 130 are determined solely by the WDM vendor and are “opaque” to the switches/routers 110 and 150. Thus, the WDM system 130 may be said to perform “opaque WDM.”
The conventional WDM system 130 may be termed “opaque” in the following additional sense. The multiplexer 132 and the demultiplexer 136 are both typically optical-to-electronic-to-optical (OEO) devices. The multiplexer 132, for example, converts received photons in an output 120 to an electrical signal, performs clock recovery, and generates a new optical signal for transmission down the fiber 134 using the electrical signal. Such clock recovery tends to “clean up” any (analog) imperfections in the output 120, but may introduce errors as well. As an example, if an imperfection in the output 120 is so great that a bit is incorrectly decoded (e.g., 1 as a 0 or vice versa), the multiplexer 132 may create a “clean” or full amplitude copy of the incorrect bit. The demultiplexer 136 will then receive the “clean,” but incorrect, bit without awareness of the signal imperfection that caused the incorrect decoding of the bit. The conventional WDM system 130 thus may be termed “opaque” with respect to light (i.e., photons).
FIG. 2 is a block diagram of a conventional opaque WDM system 130 that includes optical-electronic and electronic-optical devices, such as receivers 210, transmitters 220, receivers 260, and transmitters 270. The WDM system 130 also includes a coupler 230 connected to a splitter 250 by an optical path 240. The n-channel WDM system 130 receives data from n separate physical interfaces 205, each carrying a data signal. Typically these interfaces 205 are fiber interfaces carrying SONET signals. The interfaces may alternatively be gigabit Ethernet interfaces or other types of interfaces. Receivers 210 perform optical-to-electronic (OE) signal conversion, as well as analog-to-digital (AD) conversion. The receivers 210 terminate the SONET section, Ethernet segment, etc., and convert modulated light pulses into digital, electronic information 215.
The transmitters 220 modulate the digital, electronic information 215 onto separate wavelengths of light. Each of the transmitters 220 converts the electronic digital information 215 to an optical analog signal 225, using its own laser. The lasers in the transmitters 220 may be either directly modulated or externally modulated. All the different analog signals 225 are coupled, and possibly amplified, by the optical coupler 230 into the optical path 240 for wide-area transmission.
On the receiving side of the optical path 240, the splitter 250 separates the received optical signal into its n component wavelengths 255. The splitter 250 passes each of the n wavelengths 255 to a receiver 260. Each of the receivers 260 demodulates its optical signal to recover the digital information 265 contained therein. The transmitters 270 transmit the recovered digital information 265 on their own separate physical ports 275. Although only one direction is shown in FIG. 2, typically sending and receiving systems 130 are deployed symmetrically. Hence, there would be an additional sending and receiving system 130 transmitting in the opposite direction.
The analog-to-digital and optical-to-electronic portions of the WDM system 130 are major contributors to its high cost. Additionally, these portions require upgrading every time that signaling speeds increase or formats change. When transmission speeds increase (for example, from OC12 to OC48 to OC192) or new protocols are to be supported, the receivers 210, the transmitters 220, the receivers 260, and the transmitters 270 have to be upgraded.
The problems inherent in this conventional architecture are several. The system requires one laser per wavelength. Demodulation and remodulation (optical-electrical-optical) within the WDM system is expensive. Also, WDM equipment is protocol-specific (i.e., SONET, Gigabit Ethernet, etc.), and each such standard protocol needs to be supported individually by the WDM equipment. Further, as noted above upgrades are troublesome due to their extensive nature.
As a result, a need exists for a WDM system that does not require OE conversion and that can readily support multiple protocols from attached switches and routers.